The technical field of this invention is that of nondestructive materials characterization, particularly quantitative, model-based characterization of surface, near-surface, and bulk material condition for flat and curved parts or components. Characterization of bulk material condition includes (1) measurement of changes in material state, i.e., degradation/damage caused by fatigue damage, creep damage, thermal exposure, or plastic deformation; (2) assessment of residual stresses and applied loads; and (3) assessment of processing-related conditions, for example from aggressive grinding, shot peening, roll burnishing, thermal-spray coating, welding or heat treatment. It also includes measurements characterizing material, such as alloy type, and material states, such as porosity and temperature. Characterization of surface and near-surface conditions includes measurements of surface roughness, displacement or changes in relative position, coating thickness, temperature and coating condition. Each of these includes detection of electromagnetic property changes associated with either microstructural and/or compositional changes, or electronic structure (e.g., Fermi surface) or magnetic structure (e.g., domain orientation) changes, or with single or multiple cracks, cracks or stress variations in magnitude, orientation or distribution.
Common methods for measuring the material properties use interrogating fields, such as electric, magnetic, thermal or acoustic fields. The type of field to be used depends upon on the nominal properties of the test material and the condition of interest, such as the depth and location of any features or defects. For relatively complicated heterogeneous materials, such as layered media, each layer typically has different properties so that multiple methods are used to characterize the entire material. However, when successively applying each method, there is no guarantee that each sensor is placed at the same distance to the surface or that the same areal material region is being tested with each method without careful registration of each sensor.
A relevant example is the condition of thermal barrier coatings (TBCs) used on critical engine components in the turbine section. Because of the severe environment in which they operate, turbine section components are subject to a variety of damage mechanisms during their service lives. Their typical operating temperature of up to 2000° F. causes migration of alloying elements and accelerates inward diffusion of oxygen. Oxidation of the bond coat can introduce substantial stress at the bond coat/ceramic interface and result in spallation of the ceramic top coat. The combination of this high operating temperature and the high mass flow rate experienced by the turbine blades results in thinning of the top coat, which can produce hot spots in the underlying blade or vane resulting in nonuniform degradation of the thermal barrier coating. Realistic assessment of the TBC condition, based on measured intrinsic properties can significantly reduce the life-cycle costs associated with these components.
Characterization of these coatings poses challenges because of the large number of variables associated with their layered construct. Variations in the properties and/or dimensions of the coatings or substrate have the potential to obscure other conditions. Characterization of TBCs and bond coats with conventional eddy current sensors is impractical due to the lack of sensor reproducibility, the difficulty in modeling the complex winding interactions with layered media, and the complex shape of turbine blade surfaces.
Conventional eddy-current sensing involves the excitation of a conducting winding, the primary, with an electric current source of prescribed frequency. This produces a time-varying magnetic field, which in turn is detected with a sensing winding, the secondary. The spatial distribution of the magnetic field and the field measured by the secondary is influenced by the proximity and physical properties (electrical conductivity and magnetic permeability) of nearby materials. When the sensor is intentionally placed in close proximity to a test material, the physical properties of the material can be deduced from measurements of the impedance between the primary and secondary windings. Traditionally, scanning of eddy-current sensors across the material surface is then used to detect flaws, such as cracks.
Another example is the measuring of applied and residual stresses and detecting early stage fatigue damage. Highly stressed aircraft components, such as landing gear components, require the use of steels such as 4340 M and 300 M heat treated to very high strength levels. The integrity of these components is critical to the safe operation of aircraft and for maintaining readiness of military aircraft. However, unintentional loading of these components, such as a hard landing or during towing or taxiing, can impart residual stresses that compromise the integrity of the component.
Existing magnetic/electromagnetic, diffraction, ultrasonic and other methods for assessment of residual stresses in steel components or monitoring of applied stress over wide areas are not yet practical or cost-effective. Typically, discrete strain gages are mounted directly onto the material under test (MUT). However this requires intimate fixed contact between the strain gage and the MUT and individual connections to each of the strain gages, both of which limit the potential usefulness for monitoring stress over large areas. Furthermore, strain gages are limited in durability and do not always provide sufficient warning of gage failure or malfunction. Correlations between magnetic properties and stresses in ferromagnetic materials have been studied for over 100 years. Magnetostriction effect data suggests that, depending on the magnitude and sign of the magnetostriction coefficient, correlation between stress and magnetic permeability within certain ranges of the magnetic field should be present. However, attempts to use conventional inductive, i.e., eddy-current sensors for assessment of residual stresses as well as for a number of other applications have shown significant limitations, particularly for complex geometry components.